Designing a 24-100V to 12V Buck Converter Circuit

[Baluncore]

Ok I will check and update what this project is for, but some safety features like short circuit planned to implement. But I study in detail as well.

You need an input voltage below 50 volts to be low voltage.
You will not receive help so long as the input voltage is above 50 V.
You must respecify the project.

 

[DaveE]

You need an input voltage below 50 volts to be low voltage.
You will not receive help so long as the input voltage is above 50 V.
You must respecify the project.

Yes.

The limits I'm used to (from EN60950-1 et.al.) are 60Vdc or 42.4Vac (which is 60V peak).

Granted this seems like pointless nit picking. It is, but that's not my point. My point is that designers need to know the relevant safety standards for their application. If you don't, you need to stop and find out. Medical? Patient contact? Industrial machines? Autos? Household appliances? IT equipment? Lab equipment? Military equipment? Mining?... They all have safety standards.

We don't know yours, nor is it our responsibility. I used to bill at $100/hour for that sort of advice (it'd be more now, a decade later).

My big gripe is that this is never mentioned in schools. I don't expect that they actually teach the standards, that's a waste of effort, they are all a little different and they'll change during your career anyway. Maybe teach some general concepts of design for safety. But, for god's sake, engineers should know they exist from day one and that they'll have to figure out and read the ones that matter to them.

PS: My other big gripe are the MBA types that will hire anybody to do the work as quickly as possible. "OK, maybe you're really a chemist, or a microcontroller guy, but you can also do our power supplies, because I don't want to hire specialists". This is why you'll be better off buying a Toyota than a Yugo. This is why we have lawyers. Don't feed the lawyers.

\EndOfRant

 

[PhysicsTest]

I saw the data sheet of the LTC3703 and there was a design example on page 27, i followed the same procedure.
LTC3703 data sheet
Specifications:
VIN = 24V to 100V, VOUT = 12V, IOUT(Max) = 10A, f = 250kHz

1. RSET
As per the formula in the data sheet
RSET(kOhm) = 7100/(250 - 25) = 31.5KOhm -> eq1

2. Inductor value for about 40% ripple current at maximum VIN
L = 12/250K*0.4*10(1 - 12/100) = 10.56uH. -> eq2

3. Minimum On-time
ton(min) = 12/(100*250kHz) = 480nS -> eq3
which is above LTC3703's 200ns Minimum on-time.

4. Top and Bottom MOSFET switch

Since the drain of each MOSFET will see the full supply voltage 100V (max) plus any ringing, choose a 125V MOSFET to provide a margin of safety.

PMAIN = 1.32W -> eq4

= 96.4DegC.

=1.84 -> eq5
TJ = 70 + 1.84*20 = 106.8

5. Current Limit Resistor

RDS(ON) = (25mOhm)/2 [1 + 0.009(105 - 25)] = 21.5mOhm
IMAX pin voltage = 10*0.0215 = 0.215V
RMAX = 0.215/12uA = 18k -> eq6

6. CIN is chosen for an RMS current rating of 5A (IMAX/2) at 85DegC.
7. For the output capacitor 2 low ESR OS-CON capacitors (18mF) are used.

This is how i am approaching, i need to select the MOSFET as per the specifications and started drawing the design in LTSPICE.